AT395312B - METHOD FOR RECOVERY OR RECOVERY OF ACID FROM METAL SOLUTIONS OF THIS ACID - Google Patents

Description

AT395312B

The invention relates to a process for the recovery or recovery of acids from metal-containing solutions by spray roasting of the solutions and subsequent absorption and / or condensation of the gases formed in an aqueous absorption solution, the resulting solids being drawn off.

Such solutions, in particular solutions containing nitric acid or a mixture of nitric and hydrofluoric acid, fall in the surface treatment (pickling, etching, chemical milling) of steels, stainless steels (chromium and chromium-nickel steels), special alloys (eg nickel-based alloys) and special metals (e.g. titanium, zircon, tantalum) and when dissolving these metals or their compounds in the course of metallurgical processes. Accordingly, these solutions contain the ions of the above-mentioned metals as well as other alloy elements or impurities present in small amounts. These solutions must be discarded when a certain metal content is reached, which - depending on the pickled alloy - is 20 to 60 g / 1 dissolved metal. Such solutions are usually disposed of by neutralization. This represents a not inconsiderable environmental problem due to the large amounts of sludge and nitrate pollution in the wastewater. In addition, the operation also results in a high expenditure for acid purchases and for neutralization chemicals. For this reason, processes have been described in the literature for some years which at least partially enable the recycling of the chemicals mentioned. The disadvantage of all these processes is that they can usually only process a certain waste solution, usually a solution of Fe, Cr and Ni in a mixture of hydrofluoric acid and nitric acid, such as is obtained when treating stainless steels. In addition, in many processes, the metals contained in the solution are again generated as neutralization sludge, which does not solve the landfill problem.

In general, the processes mentioned can be divided into distillation, crystallization, extraction, ion exchange, dialysis and spray roasting processes. The spray roasting process has been used for years for the complete recovery of hydrochloric acid, such as is used for carbon pickling. The used pickling solution is sprayed into a reactor and brought into contact with hot gases that are generated by a burner protruding into the reactor. This leads to a complete decomposition of the solution: 2 FeCl2 + 2 H20 + 1/2 02 = Fe ^ + 4 HCl

The metal content of the solution is obtained as metal oxide, and the HCl gas formed is again absorbed into hydrochloric acid in water. In order to achieve the decomposition described above, temperatures of about 500 to 1000 ° C. in the reactor typically have to be reached. This and similar processes have so far only been used for the recovery of hydrochloric acid. There was a prejudice against the use of this process for the recovery of other acids (e.g. hydrofluoric acid or nitric acid), since it was feared that the nitric acid would decompose strongly into NO at the temperatures mentioned above. that is difficult to absorb. Corrosion problems were also feared at these temperatures.

In AT-PS 376 632 only the recovery of hydrofluoric acid from heavy metal fluorides and solutions containing them, such as. B. nitric hydrofluoric acid pickling described.

This process is based on heavy metal fluorides as solid roasted material, and therefore requires a pretreatment of the solutions containing the metals and acids. The solid roasted material is introduced into a rotary tube furnace via a screw conveyor, which is heated from the outside by means of a burner and in which it is brought into contact with specially supplied water vapor. The roasting process takes place at 200-800 ° C.

However, if the temperatures are reduced to about 500 ° C., the dwell times of the solid roasted material in the rotary tube furnace are at least 5-6, preferably 8-10 minutes, which results in considerable energy consumption. In addition, hydrogen vapor must be made available for the reaction, which requires further use of energy and additional plants.

The object of the invention is therefore to provide a process for the recovery or recovery of nitric acid or a nitric-hydrofluoric acid mixture from metal-containing solutions of this acid (s) while overcoming the existing prejudices when using the spray roasting process and the absorption of the exhaust gases for this purpose. Surprisingly, it was found that solutions which contain metal ions and the said acids can be decomposed at much lower temperatures than is required for the above-mentioned solutions containing hydrochloric acid. The process of the type mentioned at the outset is characterized in accordance with the invention in that for the recovery or recovery of nitric acid or a nitric-hydrofluoric acid mixture from their solutions, in particular from solutions containing cations such as Fe, Cr, Ni, Ti, Zr, Al, these solutions are sprayed at a temperature of 200 to 500.degree. C., preferably 300 to 400.degree. C., and the gases formed are absorbed and / or condensed at a temperature of 0 to 70.degree.

According to the invention, the metal and acidic solutions are prepared without pretreatment or preparation -2-

AT395312B processed, which gives the east »advantage that time, energy and investment costs are saved. Instead of an externally heated rotary kiln, the conversion to metal oxides and acid molecules takes place in a reactor, where the solutions are sprayed and heated by gas burners directly to the reaction temperature. By spraying, i.e. H. the fine distribution of the raw materials and direct heating hardly cause heat loss, e.g. B. by heating the systems so that a quick and complete implementation of the sprayed amount of liquid can be done with less energy. In addition, the solutions are used as starting materials which have a sufficient water content for the reaction.

In an embodiment of the invention it is provided that the absorption and the condensation take place in two stages, the absorption and condensation in the first stage at a temperature of 20 to 70 ° C., preferably 50 to 60 ° C., and in the second stage at one Temperature of 0 to 40 ° C, preferably 20 to 30 ° C, takes place in the said temperature range, the contrary conditions that the yield increases at low temperature, on the other hand, the costs for cooling should be kept as low as possible, are best met

According to the invention, the gases formed during spray roasting can advantageously be used to warm up the solution. In contrast to the processes described above, which were developed for the recovery of hydrofluoric acid and nitric acid, the new process described below can be used to create solutions of any composition, i.e. solutions which contain any combination of the cations (Fe, Cr, Ni, Ti, Zr, Al, etc.) as well as solutions containing either only hydrofluoric acid or only nitric acid or a combination of these acids.

The metal content of the solutions is obtained in the form of dry metal oxides or metal salts, which can be reused as such or processed through further process steps to give even higher quality products (metal powder) or can be deposited without any problems.

The following chemical and physical reactions occur in the reactor during thermal decomposition: 1. H20 (l) = H20 (g); Evaporation of water 2. HN03 (aq) = HN03 (g); Nitric acid evaporation 3rd HF (aq) = HF (g); Hydrofluoric acid evaporation 4. 2 HN03 (g) = N02 (g) + NO (g) + H20 (g) + 02 (g); HN03 decomposition 4a 4 HN03 (g) = 3 N02 (g) + NO (g) + 2 H20 (g) +1.5 02 (g); HN03 decomposition 5. 2 NC> 2 (g) = 2 NO (g) + 02 (g); NC ^ decomposition 6. MeF2 (aq) + H20 (g) = MeO (s) + 2 HF (g); Fluoride decomposition 7. Me (N03) 2 + H20 (g) = MeO (s) + 2 HN03 (g); Nitrate decomposition

Reactions 6 and 7 are only to be regarded as an example (for a divalent metal cation); they must be formulated accordingly for metals of different value and for the occurrence of charged or oxygen-containing complexes. The metal oxides formed after the 6th and 7th largely collect at the bottom of the reactor and are continuously removed from there with a discharge device. Some of them are carried along with the exhaust gas stream and removed in subsequent dust separators. The exhaust gas stream is fed to an absorption or condensation system. If the solution contains only nitric acid, the absorption-condensation system can consist of only one column, otherwise at least two columns must be used. The columns are loaded with circulating absorption solution, which is cooled to a certain temperature in heat exchangers. The concentration of the acid flowing off can be influenced by the choice of the absorption or condensation temperature, as will be explained later.

The invention is described below with reference to the drawing. 1 shows a block diagram of a plant for acid recovery according to the invention, FIG. 2 shows a detail of the block diagram according to FIGS. 3 to 5 diagrams for explaining the invention. In Fig. 1 denotes (1) a reactor to which the metal-containing solution to be worked up is fed via a single-component nozzle (3) using a metering pump (2). The heating of the reactor (1) »takes place, for example, with the aid of an acetylene oxygen burner (4) attached in the vicinity of the reactor base.

At the bottom of the reactor (1) there is a discharge device (5) for the continuous discharge of the solids that are formed. From the head of the reactor (1), the hot exhaust gases are passed to a dust separator (6), in which the remaining solids are separated. The exhaust gases then arrive at an absorption condensation system, which consists, for example, of first and second columns (7), (8) connected in series. The absorption liquid (water) is removed from the bottom of each column (7), (8), each with a »pump (9) , (10) suctioned off and returned to the top of each column (7), (8) via a respective heat exchanger (11), (12). The heat exchangers (11), (12) are, for example, water-cooled. Finally, the exhaust gases emerging from the second column (8) are passed through two wet scrubbers (13), (14) connected in series and from the second -3-

AT395 312B

Wet scrubber (14) suctioned off with a blower (15). In the variant shown in FIG. 2, the solution to be worked up is preheated in a heat exchanger (16) by the hot exhaust gases emerging from the reactor (1), as a result of which energy can be saved. As mentioned, the acid concentration in the first and second columns (7), (8) can be influenced by the choice of the absorption or condensation temperature. 3 and 5 show the possible variations when z. B. the second column (8) is kept constant at 20 ° C, while the temperature in the first column (7) is varied from 40 ° C to 60 ° C. It is shown that operating conditions are possible in which a mixture of hydrofluoric acid and nitric acid is obtained in the first column (7), while a nitric acid with only a very low HF content is formed in the second column (8). This is in connection with surface treatment of metals is particularly important, since certain pickling processes are carried out with a hydrofluoric acid / nitric acid mixture, others with pure nitric acid. 3 shows the relationship between the total volume of the continuous solution and the temperature. It is also shown that the hydrofluoric acid obtained predominantly contains (95%) in the outlet from the first column (7) (95%), to a considerable extent in the outlet from the second column (8) (4%) and to a very small extent in the oxide is (fluoride content of the oxide: 02 to 0.02%). The nitric acid has to be recovered by absorption of the nitrous gases. First, only the portion present as NO2 is converted back into nitric acid after the following reaction: 8. 3 NO 2 (g) + H 2 O (1) = 2 HNOs (aq) + NO (g); absorption

The primary NO present in the gas mixture as well as the NO portion resulting from the absorption reaction (8.) must be oxidized to enable further absorption after the following reaction: 9. 2 NO (g) + 02 (g) = 2 N02 (g ); oxidation

The reactions 8th and 9th are widely described in the literature, they form the basis of nitric acid production and are therefore state of the art. It can be seen that the waste gases resulting from the decomposition of the metal solution in the reactor have a composition with respect to the NO 2 / NO ratio which enables recovery of approximately 33% of the nitric acid originally used in the first column. This corresponds to a degree of oxidation of the waste gases of 50% according to a decomposition reaction (4.).

By means of an oxidation tower (not shown) placed between the first column (7) and the second column (8), which leads to the prolongation of the residence time, a sufficient degree of oxidation is achieved in order to again in the second column (8) 40% of the nitric acid used to absorb. In addition, an increase in the nitric acid yield after reaction 10 can be achieved by adding an oxidizing agent such as hydrogen peroxide. The metering can be carried out either in the first column (7) or in the second column (8) or else in a separate scrubber. Finally, a combination of these variants is also possible.

The reaction follows: 10. N0 + H202 = N02 + H20

The results of practical embodiments of the invention are described below. Example 1:

The solution obtained when stripping stainless steel with a mixture of nitric acid and hydrofluoric acid.

A total of 16.36 liters of a solution having the following composition was sprayed into the reactor over 5.5 hours: hno3 ................ .......... ...... 318.0 g / 1 HF ...................... ............... .. 35.8 g / 1 Fe ........................ ................. 36.3 g / 1 Cr ........................ ................. 19, 8 g / 1 Ni ........................ ................. 25.2 g /1

The task was carried out with the metering pump (2) but the single-component nozzle (3). The reactor (1) was heated with the aid of the acetylene oxygen burner (4) located near the bottom of the reactor. The temperature above the burner level was 390 to 470 ° C, in the flue gas 140 to 160 ° C.

The entire system was sucked off with the blower (14), the suction amount being 12.56 nr / h. The 4-

AT 395 312 B

Exhaust gases were first passed through the dust separator (6) and then through the two columns (7) and (8) connected in series, which were filled with pall rings. With the help of the circulation pumps (9) and (10), the columns (7), (8) were charged with absorption solution. The temperature of the circulating solution could be kept at a certain level with the help of the heat exchangers (11) and (12). The temperature was 50 ° C in the first column (7) and 22 ° C in the second column (8). The absorption solution was water at the beginning of the test, and the acidity of the solution increased with the progress of the test.

Finally, the gas was passed through the two wet scrubbers (13) and (14) arranged one behind the other

At the end of the test, samples were taken from columns (7), (8) and wet scrubbers (13), (14) and analyzed for HNOg and HF. During the test operation, the ΝΟχ and HF content of the exhaust gas was determined using Dräger tubes Analysis results and the known volumes of the individual devices were able to draw up a volume balance. Furthermore, the oxide formed from the reactor (1) and the dust collector (6) was collected, weighed and analyzed. At the end of the test, 33.0% of the total amount of HNO3 added was in the first column (7) or in the overflow of the first column (7) 43.2% in the second column (8) or in the overflow of the second column ( 8) 7.2% in the first wet scrubber (13) 3.0% in the second wet scrubber (14) 113% in the exhaust gas 0% in the oxide 97.9% total

At the end of the test, 86.0% of the total amount of HF was in the first column (7) or in the overflow of the first column (7) 9.6% in the second column (8) or in the overflow of the second column ( 8) 1.9% in the first wet scrubber (13) 0% in the second wet scrubber (14) 0% in the exhaust gas 13% in the oxide 98.8% total

The deviations of the sums of 100% result from the inaccuracies in the analysis.

Example 2:

The solution obtained when stripping stainless steel with a mixture of nitric acid and hydrofluoric acid and ammonium nitrate.

The experiment was carried out with the same solution as in Example 1, but ammonium nitrate was added to the solution, so that a content of 15 g / l NH4NO3 resulted. As described in Example 1, an overall balance was drawn up over the experiment. It was found that the addition of NH4NO3 did not lead to an increase in the nitrate values in the recovered acid and no ammonium could be detected. This justifies the assumption that the ammonium nitrate is completely decomposed after one of the reactions below: 11a. NH4NO3 = N20 + 2 H20 1 lb. NH4NO3 = N2 + 2H20 + 1/202

This fact is important because pickling solutions are sometimes mixed with urea ((NH2) 2CO) in order to reduce the undesirable formation of nitrogen oxides during metal dissolution. This sometimes results in the formation of ammonium nitrate, which eventually reaches the wastewater via the rinse water or the neutralization of the pickling acid and can lead to unacceptably high ammonium concentrations there. The experiment described above shows that ammonium nitrate can be removed from the circuit and destroyed using this method.

Example 3:

The solution obtained when pickling pure titanium with nitric acid and ussic acid with a small admixture of sulfuric acid. -5-

AT 395 312 B

Analogous to that described in Example 1, a total of 12.291 of a solution with the following composition was applied over 9 hours:

Ti ................... .......... 32.0 g / 1 HN03 ............. ......... 125.4 gd HF ................. .......... 34.8 g / 1% so4. .......... .......... 5.0 g / 1

The temperature was 390 ° C. in the burner level, 45 ° C. in the first column (7) and 35 ° C. in the second column (8).

After the end of the experiment, analysis was carried out as in Example 1. Of the total amount of HNO3 added, 33.0% were in the first column (7) or 41.4% in the overflow of the first column (7) in the second column (8) or 7.0% in the overflow of the second column (8) first wet scrubber (13) 3.0% in the second wet scrubber (14) 14.5% in the exhaust gas 0% in the oxide 98.9% total

At the end of the experiment, 98.5% of the total amount of HF added was in the first column (7) or in the overflow of the first column (7) 2.9% in the second column (8) or in the overflow of the second column ( 8) 0% in the first wet scrubber (13) 0% in the second wet scrubber (14) 0% in the waste gas 0.1% in the oxide 101.5% total

At the end of the test, 58.0% of the total amount of H2SO4 added was in the first column (7) or, in the overflow of the first column (7), 42.0% in the oxide 100.0% total

Example 4:

The solution that results from pickling pure titanium with nitric acid and hydrofluoric acid with a small amount of sulfuric acid, plus the addition of peroxide.

The experiment was carried out with the same solution as in Example 3, but a solution of hydrogen peroxide was added to the second column (8). The balance showed an increased nitric acid yield in this column and a reduction in the NOx content in the exhaust gas:

The total amount of HNO3 used was: 33.2% in the first column (7) or in the overflow of the first column (7) 46.5% in the second column (8) or in the overflow of the second column (8) 8.2% in the first wet scrubber (13) 3.0% in the second wet scrubber (14) 9.5% in the exhaust gas 0% in the oxide 100.4% Total H2O2 was added in a stoichiometric ratio to NO (corresponding to reaction 10), the ΝΟχ in the exhaust gas was assumed to be 100% NO. As can be seen from the above examples, other acids, such as. As sulfuric acid, hydrochloric acid, phosphoric acid, and other chemical compounds, such as. B. ammonium salts or organic substances. -6-

Claims (4)

AT395312B PATENT CLAIMS 1. Process for the recovery or recovery of acids from metal-containing solutions by spray roasting of the solutions and subsequent absorption and / or condensation of the gases formed in an aqueous absorption solution, the resulting solids being stripped, characterized in that for recovery or Recovery of nitric acid or a nitric-hydrofluoric acid mixture from their solutions, in particular from solutions containing cations such as Fe, Cr, Ni, Ti, Zr, Al, these solutions at a temperature of 200 to 500 ° C, preferably 300 to 400 ° C, sprayed and the gases formed thereby are absorbed and / or condensed at a temperature of 0 to 70 ° C. 2. The method according to claim 1, characterized in that the absorption and the condensation takes place in two stages, the absorption and condensation in the first stage at a temperature of 20 to 70 ° C, preferably 50 to 60 ° C, and in the second stage at a temperature of 0 to 40 ° C, preferably 20 to 30 ° C, takes place. 3. The method according to claim 1 or 2, characterized in that an oxidizing agent is added during absorption and condensation. 4. The method according to claim 3, characterized in that hydrogen peroxide is used as the oxidizing agent. For this 2 sheets of drawings -7-